An increase in speed of a multilayer printed wiring board to be used in a computer or peripheral device has progressed in recent years in association with an increase in speed of a semiconductor device. Further, there has been a growing request for high-speed, large-capacity transmission by a communication instrument or broadcasting instrument in association with rapid widespread of the internet and mobile phones. In view of the foregoing, an improvement in high-frequency characteristic has been needed in such multilayer printed wiring board. In particular, attention has been paid on problems involved in an advanced multilayer printed wiring board to be used in a high-frequency region exceeding 1 GHz such as transmission loss and circuit delay.
A glass cloth using E glass fibers is generally used as a base material conventionally used in a printed wiring board. A laminated plate is produced by impregnating the glass cloth with a thermosetting resin such as an epoxy resin or a phenol resin and molding the resultant under heat and pressure.
However, a low dielectric constant and a low dielectric loss tangent are requested of a high-frequency printed wiring board in which a high-speed semiconductor device is implemented. In view of the foregoing, a D glass fiber (Patent Document 1), thermosetting PPE resin, or the like excellent in electrical characteristics has started to be used instead of the conventional combination of an E glass fiber and an epoxy resin or phenol resin, or the like.
However, the D glass fiber has currently become obsolete owing to its problems described below: the fiber is poor in productivity owing to its high melting temperature, and, furthermore, the fiber is poor in drilling property when turned into a printed wiring board because the fiber has a high SiO2 content. In view of the foregoing, an NE glass fiber having drilling property comparable to that of an E glass fiber while maintaining a low dielectric constant and a low dielectric loss tangent has been developed. However, an additional increase in frequency at which a printed wiring board can be used has been advanced, and a base material for a printed wiring board more excellent in electrical characteristics than the NE glass fiber has been requested.
In addition, a printed wiring board composed of a fluorine-based resin has been conventionally used as a high-frequency printed wiring board in many cases. In addition, the fluorine-based resin board has been used as a single-sided board or double-sided board in most of the cases because it is difficult to turn the board into multiple layers. However, there has been a growing request for an increase in number of layers even in a high-frequency printed wiring board these days.
A quartz glass fiber having the lowest relative dielectric constant and the lowest dielectric loss tangent out of the glass fibers is expected to be a base material for a printed wiring board, and an investigation has been conducted on the introduction of the fiber. However, the quartz glass fiber involves the following problem: the fiber is so hard that it is difficult to subject a multilayer printed wiring board composed of the fiber to boring with a drill or cutting. In view of the foregoing, a proposal has been made for improving the drilling property of the fiber (Patent Document 2). However, a quartz glass fiber improved as a result of the proposal is so expensive as to find use only in a few special applications.
In addition, an invention concerning a woven fabric for a printed wiring board composed of E glass, D glass, and silica fibers, and polytetrafluoroethylene fibers has been disclosed (Patent Document 3). Although the polytetrafluoroethylene fibers used in the woven fabric are each excellent in dielectric characteristic, and each have high heat resistance, the fibers are expensive, so limitations are imposed on the use of the fibers in a general-purpose printed wiring board as in the case of a quartz glass cloth. As a result, the fibers find use only in a few special applications.
A method of implementing a semiconductor device in a printed wiring board has been changing in association with an increase in speed of the device. A wire bonding method has been conventionally employed, but it is difficult for the wire bonding method to correspond to the progress of: a finer wiring pitch; a finer electrode; and an increase in information processing speed. A flip chip method involving joining a device and a board with a projecting electrode called a bump has started to be employed instead of the above method.
In implementation by the flip chip method, a remarkable difference in linear expansion coefficient between the device and the printed wiring board may be responsible for a wiring failure, so the development of a low linear expansion coefficient printed wiring board corresponding to the flip chip method has been demanded.
The low linear expansion coefficient printed wiring board generally achieves its low linear expansion coefficient by being mixed with a silica-based filler at a high ratio. However, a phenomenon called a powder fall in which the filler falls from the portion at which the board is processed with a drill or the like occurs, and is perceived as a problem. In view of the foregoing, a reduction in linear expansion coefficient of a base material for a printed wiring board has been requested.
The quartz glass fiber is an example of a base material having a low linear expansion coefficient, but some problems must be solved owing to the above reasons.
Further, a glass cloth with its linear expansion coefficient improved by interlacing the E glass and a glass fiber having specific composition at a specific ratio has been proposed as a prior art (for example, Patent Document 4). However, the glass cloth cannot provide sufficient characteristics.
Patent Document 1: JP 63-2831 A
Patent Document 2: JP 2004-99377 A
Patent Document 3: JP 2-61131 A
Patent Document 4: JP 10-310967 A